So I'll just summarize here the things that I've already said in other places. Actually most of them are in this TALKPAGE.
Gengetsu said that his Yin Nature means that he is a genjutsu user, but this doesn't indicate that all genjutsu automatically classify as Yin Nature, because an ordinary understanding of Gengetsu's statement is that Yin Nature is a fundamental nature for genjutsu users, simple and short. A crude analogy would be Kisame saying that his Water Nature means that he is a ninjutsu user (which is true), and then we conclude that all ninjutsu partly stem from Water Nature or solely from it.
And for some reason, this statement from the databook, the proportional amount of genjutsu that involves "creating form from nothing" [i.e. Yin Release] is high, became the evidence that proved all genjutsu are Yin Release. Does that even add up? And when other information from the manga and databook contradict this claim, we simply write it off as Kishi's error. For example, Hashirama and Nagato do not have Yin Nature, yet they both used genjutsu. Circular argument at its best.
Let us all hear your thoughts.